1. Field of the Invention
This invention relates to an imaging optical system such as a photographic lens, a video lens or an industrial collimator lens, and in particular to an imaging lens of a simple construction in which chromatic aberration has been corrected in the visible wavelength range.
2. Related Background Art
In recent years, the need for making image pick-up optical systems for consumer's use such as photographic lenses and video lenses or industrial lenses simple, compact and high in performance has been rising. As the means for achieving this, application of an aspherical lens, special glass having abnormal dispersion (for example, FK01 produced by Obara Kogaku Co., Ltd.) or quartzito is known, but this has often been commercially insufficient in cost.
The optical system according to the present invention overcomes the above-noted difficulty by applying to a lens element a gradient index lens recently utilized as a part for optical communications or the reading optical system of a copying apparatus or the like.
At present, numerous methods such as the ion exchanging method, the crystal growth method and the molecular staffing method are known as a technical means for making gradient index lenses, and from the practical viewpoint, it nearly seems that the area of the index distribution is of the order of 10 and several mm and the difference in refractive index is of the order of 0.2. As an example of the application of such gradient index lens, the design of an optical system comprising two gradient index lenses of 50 mm/F.sub.2 for photographic lenses is shown in the treatise of Atkinson et al., Applied Optics, Vol 21, No. 6, 1982. However, this is an example which has an index distribution in a direction perpendicular to the optic axis, and also, in this example, the area of the variation in refractive index is great and therefore, there is a great limitation in the manufacture of base metal. Also, in Japanese Patent Application Laid-Open No. 149312/ 1984, it is proposed to use as a lens element a lens having an index distribution in the direction of the optic axis to make a great relative aperture lens of high performance for photography by the use of a blank which can be manufactured under the present situation. In this example, however, setting is made so that there is no dependency of the index gradient on wavelength and therefore, there is not included the point of vision at which the correction of chromatic aberration in the aberration area is positively effected, and the practical use of the index distribution as a lens for long focus is not intended. On the other hand, FIG. 7 of the accompanying drawings shows an example of the prior art which uses a lens uniform in refractive index, and the lens data of this lens are shown in Table 1 below, and the spherical aberrations of four wavelengths for showing the imaging characteristic are shown in FIG. 8 of the accompanying drawings.
TABLE 1 ______________________________________ f = 200 mm F No. 1:4 Object infinity point ______________________________________ R1 = 121.81975 R2 = -87.69947 R3 = -245.989 D1 = 14.0 D2 = 7.0 Refractive index of 1st lens Nd.sub.1 = 1.50977 Abb number .nu.d.sub.1 = 62.1 Refractive index of 2nd lens Nd.sub.2 = 1.67270 Abb number .nu.d.sub.2 = 32.1 ______________________________________
As can be seen from FIG. 2 of the accompanying drawings, as a feature of the imaging characteristic of this type, the refractive power for a light having a short wavelength becomes greater in the joined surface R2 as the incidence height, i.e., the distance of the incident light ray from the optic axis, become greater. As a result, during the aberration correction, a correction balance in which paraxial chromatic aberration is under-corrected and marginal light is over-corrected must unavoidably be selected, and the number of lenses must unavoidably be increased to make the imaging characteristic for white light good.